Conventional noninvasive flow rate meters must be fixedly clamped onto pipes or pipelines, so that the ultrasound generated by an excitation transducer can be coupled into the pipe. Neither the fixed clamping of the device nor the subsequent operation impair the media transport in the pipe, because of which this is referred to as a noninvasive flow measurement. Often, additional wedge-shaped acoustic couplers are used, which are arranged between piezoelement and object and which couple the acoustic signal output by an excitation transducer, often with additional use of a coupling medium, into the line wall.
Installing and putting into operation the excitation and receiving transducers on the pipe represents a critical work step, since a spacing between excitation and receiving transducers, which is defined depending on the medium, the wall thickness, and the material, for the ultrasonic waves generated in the pipe must be exactly maintained. Even slight deviations from the optimum spacing worsen the signal significantly. In addition, the conventional ultrasonic flow rate meters are only usable in a comparatively narrow temperature range, wherein this means the temperature of the object, preferably a pipe or a pipeline. The operating temperature of the transducer must be significantly less than the Curie temperature thereof, which is typically between 150° and 350° for a variety of employed transducers. A solution for overcoming this problem is described, for example, in DE 4124692 A1, in which the transducer is implemented using a special piezoceramic for high-temperature applications. Flow velocities or flow rates of media in objects having temperatures up to 180° may be measured in this way. In addition to the temperature problems, however, critical problems may sometimes arise because of tensions in the material of the acoustic coupler, which are generated by strong temperature gradients. Furthermore, aging of the coupling medium, which is typically gel-like, used between acoustic coupler and pipe is problematic. The signal quality of the ultrasonic wave generated in the object decreases with increasing age of the coupling medium.